Ahn Goo Choo

Near field measurements of optical channel waveguides and directional couplers

Near field microscopy is used to investigate the guided mode intensity distribution of optical channel waveguides and directional couplers with subwavelength spatial resolution. The directional coupler consisted of two single mode optical ridge channel waveguides formed with silicon nitride deposited on a lower cladding layer of SiO2 on a silicon substrate. A near field measurement of the guided mode intensity profile in the transverse direction parallel to the waveguide surface was performed across one of the optical channel waveguides. These variations are compared with model calculations. Similar transverse measurements of light propagating through a directional coupler were performed at many locations along the coupler, providing a view of the evolution of optical power transfer.

Optical channel waveguides in AlGaAs multiple-quantum-well structures formed by focused ion-beam-induced compositional mixing

Optical channel waveguiding in a AlGaAs multiple-quantum-well structure was demonstrated in a channel formed by compositional mixing induced by focused ion beam (FIB) implantation. Selective mixing was achieved by FIB implanting Si/sup ++/ with a dose of 5*10/sup 14/ cm/sup -2/ followed by rapid thermal annealing at 950 degrees C for 10 s. Raman microprobe spectra were used to characterize the lateral variation of compositional mixing. Channel waveguide loss of 17.2 dB/cm was measured, compared to 10-12 dB/cm measured for planar waveguiding. Mode field pattern measurements indicate that a change in effective index of 2.7*10/sup -4/ was induced, corresponding to an approximate mixing depth of 270 nm.<<ETX>>

Photon scanning tunneling microscopy of optical channel waveguides

Abstract Photon scanning tunneling microscopy (PSTM) has been used to characterize optical channel waveguides. Both He Ne laser and semiconductor diode laser wavelengths are utilized in the measurement of the evanescent field intensity associated with the propagating modes of optical waveguides. A tapered optical fiber tip, and also a semiconductor heterostructure tip attached to an optical fiber are employed for detection. Local values of effective refractive index are calculated from measurements of both TE and TM polarizations and compared to model calculations.

Optimization of diamond growth with statistical experiment design in an electron cyclotron resonance plasma

Multivariable statistically designed experiments have been used to optimize diamond growth in a H2‐CH4‐O2 magnetoactive plasma at 1–3 Torr total pressure. The input process parameters included total pressure, microwave power, substrate temperature, methane and oxygen fractions, and applied substrate bias, and the experimental response outputs were diamond film growth rate and crystallinity. A clearly defined characteristic 1333 cm−1 Raman peak with a 5 cm−1 full width at half‐maximum identified the optimum diamond crystal growth conditions of H2 (75%)‐CH4 (13%)‐O2 (12%), 3 Torr total pressure, and 650 °C substrate temperature.

Enhanced photoluminescence from AlGaAs/GaAs superlattice gratings fabricated by Si FIB implantation

Results are presented on the fabrication of optical gratings on an Al 0.3 Ga 0.7 As/GaAs superlattice (SL) with equal 3.5 nm barrier and well widths, by using locally FIB-enhanced mixing. As the first step, the mechanism of the mixing was studied. Si ++ was accelerated to 50 kV and lOOkV and implanted at doses ranging from 10 13 to 10 15 /cm 2 . A rapid thermal anneal of 10 s at 950°C was utilized. The average Al inter-diffusion coefficient and length were calculated as a function of FIB dose from SIMS depth profiling. The mixing was significantly enhanced by the FIB implantation. The ion dose as low as l×10 14 /cm 2 followed by RTA yields a mixing parameter of ∼90% and results in a two-order of magnitude increase in the diffusion coefficient, to a value of 4.5×10 −14 cm 2 /sec, in contrast to 1.3×10 −16 cm 2 /sec from RTA-only. The maximum mixing occurred in the region where neither Si ions nor vacancies have their maximum concentration. Instead, it coincides with the location of the positive maximum of the second derivative of the vacancy concentration profile. This fact suggests that in the time frame of RTA and with low dose, the diffusion of nonequilibrium point defects plays a major role in the process of enhancing Al-Ga interdiffusion. DBR optical gratings, consisting of thousands of spacing lines with 350nm period, were fabricated with a lOOkV FIB dose of 2×10 13 andl×10 14 /cm 2 . Photoluminescence (PL) spectra were taken from the grating region as well as the unimplanted superlattice region. The PL intensity from cavity region of the DBR was about 16 times higher than that from the original SL. This PL enhancement was verified to occur in the cavity region only by spatially scanning over the entire sample. A possible mechanism for this PL enhancement is optical feedback provided by the gratings.

Surface Morphology of Ingap in the Ai-Free Pump Ld

We have studied the effects of growth parameters and substrate orientations on InGaP quality using Normarski microscopy, photoluminescence (PL) spectrum and atomic force microscopy (AFM). The full width at half maximum (FWHNM) and peak position of PL spectrum were closely related with the surface morphology. The InGaP layers of narrower FWHM and shorter peak wavelength had smoother surface morphology. The InGaP layers grown on (100) substrates at the moderately low reactor pressure showed rougher surface than those on the tilted substrates. But the surface morphology was noticeably improved to be mirror-like at the lower reactor pressure. The surface morphology was inverted between the exact and tilted substrates in this reactor pressure. Furthermore, the samples grown on the tilted substrate exhibited rougher surface than the samples grown on the exact substrate. (111)B-misoriented growth surfaces had smoother than (111)A-misoriented surfaces.

Near-field measurements of optical channel waveguide structures

Near field scanning optical microscopy (NSOM) has been used to investigate the guided mode intensity distribution in optical channel waveguides, phase-matched directional couplers, and symmetric Y- junctions. A near field measurement of the lateral guided mode intensity profile was performed across the optical channel waveguide, and compared with model calculations. The near field guided mode intensity profiles above the waveguides were measured as a function of distance along both a directional coupler and a Y-junction, providing a near field view of the spatial evolution of optical power in these structures.

Raman Scattering Characterization of Ultrathin Films of β-SiC

Raman scattering has been used to characterize ultrathin films of β-SiC, ranging in thickness from 38 nm to 240 nm. These films were prepared on the surface of a Si substrate by a carbonization process at a temperature of 1300°C. In each case, the LO phonon near 970 cm −1 and the TO phonon near 795 cm −1 are observed, indicating the formation of β-SiC crystal. The Raman linewidths and peak positions indicate evidence of nonuniform stress and disorder. The Raman intensity of the TO phonon is nearly twice the intensity of the LO phonon measured both with and without the Si substrate, which indicates that the crystal growth was not entirely confined in the direction.

Raman and Photoluminescence Characterization of FIB Patterned AIGaAs/GaAs Multiple Quantum Wells

Raman and photoluminescence (PL) spectra have been used to characterize A1 0.3 Ga 0.7 As/GaAs multiple quantum well (MQW) structures that have been patterned by focused ion beam (FIB) implantation followed by rapid thermal annealing (RTA). Microprobe Raman scattering is used to identify the appropriate RTA and FIB implantation conditions that provide for removal of implantation-induced damage and for compositional intermixing. FIB patterned wire-like structures are characterized by spatially resolved PL spectra.

Photoreflectance characteristics of AlGaAs/GaAs structures

Room temperature photoreflectance (PR) has been used to study a series of AlGaAs/GaAs heterostructures, which have been shown by electrical measurements to contain a two- dimensional electron gas (2-DEG), with varying carrier concentrations, mobilities, and structures. Oscillatory features in the PR spectra observed above the GaAs bandgap energy cannot be simply interpreted as Franz-Keldysh oscillations (FKO).